REACTIONS OF ORGANIC
COMPOUNDS
Lecture 7 12.5 Reactions of Organic Compounds
Learning Outcomes:
At the end of the lesson the students should be able to :
Explain covalent bond cleavage: homolytic
heterolytic
Types of Covalent Bond Cleavage/Fission
All chemical reactions involved bond breaking and bond making.
Two types of covalent bond cleavage :-
Homolytic cleavage
Heterolytic cleavage
a) Homolytic Cleavage
Occurs in a non-polar bond involving two atoms of similar electronegativity.
A single bond breaks symmetrically into two equal parts, leaving each atom with one unpaired electron.
Formed free radicals.
X : X
Example:
X + X
free radicals
X X
b) Heterolytic cleavage
Occurs in a polar bond involving unequal sharing of electron pair between two atoms of different electronegativities.
A single bond breaks unsymmetrically. Both the bonding electrons are transferred
to the more electronegative atom.
Formed cation and anion.
A is more
electronegative.
B is more
electronegative.
A:- + B+ anion cation
A : B
A+ + B:- cation anion
Reaction Intermediates
a) Carbocation
b) Carbanion
c) Free Radical
They are unstable and highly reactive.
a) Carbocation
Also called carbonium ion. A very reactive species with a positive
charge on a carbon atom. Carbocation is formed in heterolytic
cleavage.
Example : (CH3)3C — Cl (CH3)3C+ + Cl-
carbocation anion
Chlorine is more electronegative than carbon and the C—Cl bond is polar.
The C—Cl bond breaks heterolitically and both the bonding electrons are transferred to chlorine atom to form anion and carbocation.
b) Carbanion
is an anion counterpart
a species with a negative charge on a carbon atom.
Carbanion is formed in heterolytic cleavage.
example:
(CH3)3C — Li (CH3)3C- + Li+
carbanion kation
b) Free Radical
~A very reactive species with an unpaired electron. ~Formed in homolytic cleavage. Examples: i)
free radicals
Cl – Cl uv Cl● + Cl ●
C C C ● + C ●
H3C H H3C ● H ● +
ii)
iii)